High strength cobalt-beryllium alloy and method of producing the same



Aug. 18, 1970 H. HATWELL ETAL 3,524,775

HIGH STRENGTH COBALT-BERYLLIUM ALLOY AND METHOD OF PRODUCING THE SAME Filed D60. 6, 1967 4 Sheets-Sheet 1 INVENTORS HENRI HATWE LL ANGEL/NE M. P. FOURDEUX ATTORNEY Aug. 18, 1970 HATWELL EI'AL 3,524,775

HIGH STRENGTH COBALT-BERYLLIUM ALLOY AND METHOD OF PRODUCING THE SAME Filed D80. 6, 1967 4 Sheets-Sheet 2 INVENTORS C H H HENRI HATWELL ANGEL! E M.P.FOURDEUX- lflzxy. 1112A ATTORNEY Aug. 18, 1970 H. HATWELL ET AL HIGH STRENGTH COBALT-BERYLLIUM ALLOY AND METHOD OF PRODUCING THE SAME Filed Dec. 6, 1967 4 Sheets-Sheet 5 INVENTORS HE NR/ HA TWE LL ANGEL lNE M. P. FOURDEUX BYM/QM ATTORNEY Aug. 18, 1970 v H, HAT-WELL ETAL 3,524,775

HIGH STRENGTH COBALT-BERYLLIUM ALLOY AND METHOD 0F PRODUCING THE SAME Filed Dec. 6, 1967 4 Sheets-Sheet 4 Kg psi x I000 I 1 l l 1 IO 2O 3O 40%ELONGATION STRAIN RATE -.42mm/mm/min.

FIG. 5.

N27 ES'J9EZ HE ANGEL/NE M. P. FOURDEUX ATTORNEY United States Patent 3,524,775 HIGH STRENGTH COBALT-BERYLLIUM ALLUY AND METHOD OF PRODUCING THE SAME Henri Hatwell and Angeline M. P. Fourdeux, Brussels,

Belgium, assignors, by mesne assignments, to Cabot Corporation, a corporation of Delaware Filed Dec. 6, 1967, Ser. No. 688,944 Int. Cl. C22f 1/10 US. Cl. 148-158 4 Claims ABSTRACT OF THE DKSCLOSURE Cobalt-beryllium alloy characterized by a periodic beryllide precipitate and having increased strength and hardness. The alloy is produced by heating to an elevated temperature, quenching the alloy and then heating the alloy to an elevated temperature to cause precipitation of the beryllide phase.

This invention relates to cobalt base alloys containing minor proportions of beryllium. More particularly this invention is directed to a beryllium containing cobalt base alloy which is characterized by substantially improved strength.

Cobalt base alloys are widely used at present, particularly in wear-resistant applications and have been considered to be very effective for this purpose. However, in an increasing number of applications, improved high strength and hardness, in addition to superior wear-resistance, has become an important consideration.

It is therefore an object of the present invention to provide a cobalt base alloy characterized by substantially improved strength and hardness.

It is another object of the present invention to provide increased strength and hardness in cobalt base alloys through the addition to the alloy of a minor proportion of beryllium.

Other objects will be apparent from the following description and claims taken in conjunction with the drawing in which FIG. 1 is a photograph showing the microstructure of a Co-Be alloy prior to its conversion into the material of the present invention;

FIG. 2 is an electron micrograph of the material of FIG. 1;

FIG. 3 is a photograph showing the characteristic microstructure of the material of the present invention;

FIGS. 4(a) and 4(b) are photographs showing, at different magnifications, the microstructure of a material having the same Co-Be proportions as that of the material of FIG. 3, but which has been prepared by a different technique; and

FIG. 5 is a graph comparatively illustrating the improved strength of the material of the present invention.

An alloy in accordance with the present invention, characterized by high strength and hardness, is a cobalt base alloy containing from about 0.9 to about 1.5 percent by weight beryllium and having a highly faulted face centered cubic matrix in which is dispersed a substantial amount of periodic beryllide precipitate, CoBe.

In the practice of the present invention a beryllium addition of from about 0.9 to about 1.5 is made to a cobalt melt and the resulting alloy is cast into an ingot and ice preferably homogenized by not working at temperatures up to about 1100 C. After homogenization, the resultant wrought alloy is heated to between 900 C. and 1100" C., the higher temperatures in this range being preferred for the higher contents of beryllium, for a time sufficient to produce a solid solution of beryllium in cobalt. From about /2 to 3 hours is ordinarily suificient for this purpose after which the alloy is rapidly quenched at a rate which is sufficient to retain beryllium in solution and which avoids significant precipitation of beryllium. Oil or Water quenching has been found to be satisfactory for this purpose.

The quenched alloy is subsequently heat treated again by heating in the range of about 500 to 800 C. for a time sufficient to cause precipitation of a dispersed beryllide phase, Co-Be, and produce a fine grained highly faulted face centered cubic matrix. Usually, a heating period of about 15 minutes at temperature is sufiicient for this pur pose with longer heating periods up to 8 hours being used for alloys containing other constituents besides cobalt and beryllium. Relatively long heating periods, e.g. 20 hours and longer are to be avoided since such treatments lead to a loss of strength in the alloy. A Co-Be phase of at least 10% by volume is produced as a result of this heating step. A cobalt beryllium alloy having the composition and structure of the material of the present invention is characterized by substantially improved strength and hardness as hereinafter shown.

With reference to the figures of the drawing, FIG. 1 (original magnification 350x) shows the relatively large grain structure which is produced in a cobalt-beryllium alloy as a result of the above first mentioned heat treatment and quenching whereby a solid solution of beryllium is achieved without significant precipitation of a beryllium phase. The alloy was a Co-l.3% Be alloy and, after homogenization, was heated for one hour at 1080 C. and quenched.

FIG. 2, an electron micrograph (original magnification 40,000X), also shows the material of FIG. 1. In this photograph the numeral 1 indicates a faulted hexagonal matrix containing stacking faults 3 which appear as closely spaced parallel bands.

FIG. 3, at a magnification of 128,000, is an electron micrograph showing the structure of the alloy of FIGS. 1 and 2 after the final heat treatment and this photomicrograph illustrates the characteristic microstructure of the alloy of the present invention. The final heat treatment for the alloy of FIG. 3 was 5 hours at 600 C. followed by quenching. The periodic beryllide preciptiate developed in the alloy as a result of this heat treatment is shown at 7 dispersed in a face centered cubic matrix 9 in which are present stacking faults 11, micro-twins 12. A hexagonal phase is present and detected by electron diffraction.

The characteristic microstructure of the alloy of the present invention can thus be seen to comprise a face centered cubic matrix, which is highly faulted as shown by the presence of stacking faults 11 and micr0twins 12, and which contains a dispersed, periodic beryllide precipitate 7 and a hexagonal phase.

The periodicity of the beryllide precipitate in the alloy of the present invention is highly important as regards the strength of the alloy and is measured as the distance between precipitate oriented in the same direction. As

can be seen from FIG. 3, the beryllide precipitate is oriented along the crystallographic in planes of the cubic matrix. For example, the precipitate, in the form of lamellae indicated generally as A, are oriented in the same direction, and can be considered to be in a plane which is substantially at right angles to the planes containing the precipitate indicated at C. For purposes of the present invention the periodicity of the beryllide precipitate can be calculated by preparing a section of alloy and obtaining the average distance between laterally adjacent precipitate which are oriented in the same direction, i.e. essentially in the same plane. For example, with reference to FIG. 3, the periodicity can be determined by averaging the distances d d d and d.;. In this particular instance, the periodicity is about 900 angstroms. In the present invention, to obtain substantially increased strength the periodicity can range from about 250 A. to about 2000 A. With increasing periodicity of the beryllide precipitate, increased ductility is obtained, however the strength of the alloy decreases with increased ductility. With a periodicity above about 2000 A., the strength of the alloy decreases rapidly. The periodicity of the beryllide precipitate can be controlled through the temperature used in the final heat treatment, with higher temperatures in the range of 500 to 800 C. providing increased periodicity, i.e., increased ductility but lower strength.

FIG. 4(a) of the drawing, shows the microstructure, at 40,000 of an alloy of Co-1.3% Be which was not prepared in accordance with the present invention and hence does not have a periodicity in the range of 250 A. to 2000 A. The periodicity of this material is about 5400 A. and, as shown hereinbelow in Example 6, does not have the superior properties of the alloy of the present invention. FIG. 4(b) at a magnification of 128,000X shows the indicated portion of FIG. 4(a) at a magnification of 40,000. Thus, the periodicity of the material of this invention (FIG. 3) can be compared directly with that of FIG. 4(b). The ultimate tensile strength of the alloy of FIG. 3 of this invention was more than 50% greater than that of the alloy of FIG. 4.

The following examples will further illustrate the present invention.

EXAMPLE I A cobalt-beryllium alloy was prepared containing 1.3% Be balance cobalt. The alloy was rolled in air at 1000- 1050 C. to about 0.22 mm. thick sheet, homogenized by heating at about 1100 C. and electrolytically polished to obtain a test specimen about 0.15 mm. thick. The test specimen was heated to 1080 C. and held at this temperature for one hour and quenched. The microstructure of this material was similar to that of FIG. 1 and is composed of about 90% HCP (hexagonal close packed) and FCC (face centered cubic). The ultimate tensile strength of the material was measured and the results are shown at I of FIG. 5.

EXAMPLE II The procedure of Example I was repeated using the same starting CoBe composition except that after quenching the material was heated to 600 C. and held at this temperature for 5 hours and quenched again. The ultimate tensile strength of this material which was in accordance with the present invention, was measured and the results are shown at II in FIG. 5. The microstructure of this material was similar to that of FIG. 3 and is composed primarily of highly-faulted FCC matrix containing beryllide lamellae. The material of this example is in accordance with the present invention and as can be seen from FIG. 5 has greatly increased strength as compared to the material of Example I having the same beryllium content.

EXAMPLE III The procedure of Example II was repeated, except that the composition of the material was 0.75% Be balance cobalt. The ultimate tensile strength of this material was about kg. per mm. at an elongation of 11%.

EXAMPLE IV The procedure of Example I was repeated using the same starting CoBe composition except that after quenching the material was heated to 700 C. and held at this temperature for 5 hours. The microstructure of this material was similar to that of FIG. 3 in accordance with the present invention. The ultimate tensile strength of this material was on the order of kg. per mm. at an elongation of 12%.

EXAMPLE V The procedure of Example I was repeated using the same starting CoBe composition except that after quenching the material was heated to 900 C. and held at this temperature for 5 hours. The microstructure of this material was similar to that of FIG. 3 in accordance with the present invention. The ultimate tensile strength of this material was on the order of 120 kg. per mm. at an elongation of 22% EXAMPLE VI The procedure of Example I was repeated using the same starting CoBe composition except that after quenching the material was heated to 700 C. and held at this temperature for 50 hours. The microstructure of this material is shown in FIG. 4. The ultimate tensile strength of the material was 106 kg. per rnmfi.

To further comparatively illustrate the improvement provided by the alloy of the present invention materials having the same CoBe proportions (1.30% Be, bal. Co) but having diiferent microstructures were prepared and tested. One of the materials referred to herein as material D had a microstructure as shown in FIG. 1 and was prepared as in Example I above by being heated for one hour at 1080 C. and oil quenched. Material of this invention, referred to as E, had a microstructure as shown in FIG. 3 and was produced by heating for one hour at 1080" C. followed by oil quenching, and a further heat treatment at 600 C. for about 5 hours as in Example II above.

The Table I below shows the improved hardness obtained in the alloy of the present invention.

TABLE I.--HARDNESS-VICKERS Material E (this invention) 425 Material D 180 As can be seen from the foregoing description and data, through the use of the disclosed heat treatment, a Co, 0.9-1.5 Be alloy having a unique microstructure is obtained which is characterized by substantially improved hardness and strength. The characteristic microstructure and hardness can be reliably developed only in the range of about 0.9-1.5% Be. It has been found that lower amounts of beryllium do not produce the desired properties of high strength and hardness whereas at higher beryllium contents due to the presence of primary beryllide compounds, the strength and hardness properties are highly errated and high strength and hardness are not consistently obtained.

The cobalt base, beryllium containing alloy of the present invention can also contain the following percentage of other constituents:

Ni Up to 30 Cr Up to 10 W Up to 5 together with minor amounts of residual elements normally present as a result of commercial melting practices.

substantial increase in the strength and hardness of the alloy.

2. An alloy in accordance with claim 1 containing about 1.3% beryllium.

3. An alloy in accordance with claim 1 containing up to 30% nickel, 10% chromium, and 5% tungsten.

4. A method for imparting increased strength and hardness to a cobalt base alloy containing from about 0.9 to about 1.5% beryllium which comprises:

(1) heating the alloy to an elevated temperature to provide a solid solution of beryllium in cobalt;

(2) quenching the alloy at a rate such that no significant precipitation of beryllium occurs; and

(3) heating the alloy to an elevated temperature to cause precipitation of a dispersed CoBe phase in the alloy;

8/1937 Hessenbruch 75-471 4/1947 Dinerstein 75-17l RICHARD O. DEAN, Primary Examiner US. Cl. X.R. 

